[css-cascade-3] Fix typo

Author: fantasai

css-cascade-3/Overview.bs

@@ -613,7 +613,7 @@ Filtering</h2>
 Cascading</h2>
 
 	The <dfn export>cascade</dfn>
-	takes a unordered list of <a>declared values</a>
+	takes an unordered list of <a>declared values</a>
 	for a given property on a given element,
 	sorts them by their declaration’s precedence as determined below,
 	and outputs a single <a>cascaded value</a>.